(1) Field of the Invention
The present invention relates to a method for improving residual stress at a welded part of a pipe made of austenitic stainless steel or the like, in which stress corrosion cracking can occur.
(2) Description of Related Art
As an example for reducing residual stress in an inner surface of a pipe, JP-A-2001-150178 discloses that an outer surface of an annular welded part is rapidly heated to decrease tensile residual stress on an inner surface. According to the document, a temperature difference between the inner and outer surfaces of the pipe is introduced by rapid heating of the pipe, so that the residual stress in the inner surface of the pipe, which is a low-temperature side, is improved by using the difference of the thermally expansion between the inner and outer surfaces.
In addition, there is a high-frequency induction heating method for decreasing tensile residual stress in an inner surface of a pipe, in which an outer surface side is heated by induction heating with use of a high-frequency induction heating coil while the inner surface of the pipe is being cooled with water, so that a temperature difference between the inner and outer surfaces are imparted.
As an example for improving tensile residual stress on an inner surface of a small diameter pipe, JP-A-2000-052247 discloses that a high-speed stream of water containing cavitation bubbles is spouted from a nozzle to the inner surface of the small-diameter pipe filled with water, and the cavitation bubbles are broken in the vicinity of the inner surface of the small-diameter pipe so that the condition of tensile residual stress is improved.
As an example for decreasing a tensile residual stress on an inner surface of a welded part of a pipe with use of an ice plug formed by cooling of the pipe from the outer surface, JP-B2-1559804 discloses that each one portion of both sides of the welded part or only the welded part is cooled from the outer surface to form the ice plug, and the welded part is isolated and consequently the inner pressure is increased, so that the vicinity of the pipe of the welded part is expanded. JP-B2-1544585 discloses an example for expanding the pipe while heating the outer surface of the pipe at the welded part to prevent a formation of the ice plug at the welded part. JP-B2-1428283 discloses an example for controlling an amount of expansion of the pipe by applying an electric current to the pipe containing a part inside of which the ice plug is formed, and accelerating a melting of the ice plug by heating. Furthermore, Michisuke Nayama, “Development of Stress Relief Method for Weld Joint of Pipe using Ice Plug”, Quarterly Journal of the Japan Welding Society, Vol. 12 No. 1, pages 132 to 136, 1994, discloses a test result of decreasing residual stress on an inner surface of a pipe with use of a ice plug formed by cooling the pipe from the outer surface.